This invention relates generally to semiconductor manufacture, and more particularly to an improved method and apparatus for aligning and attaching balls to a substrate.
Semiconductor components, such as wafers, dice and packages can include external contacts in the form of solder balls. For some components, such as chip scale packages, the balls can be arranged in a dense array, such as a ball grid array (BGA), or a fine ball grid array (FBGA). The balls provide a high input/output capability for a component, and permit the component to be surface mounted to a mating component such as a printed circuit board (PCB).
One conventional method for attaching the balls to a component substrate uses a solder reflow process. With this method the substrate can include bonding sites, such as bond pads, or land pads, on which layers of flux are deposited. A platen can be used to hold the substrate, while the flux is deposited on the bonding sites in a required pattern. After depositing the flux layers, the solder balls can be placed on the flux layers, and a convection furnace used to reflow the flux layers. After cooling, a permanent solder bond is formed between the bonding sites and solder balls.
Because the solder balls have a natural attraction for the flux layers, the alignment step is facilitated. However, one problem with this method is that during the heating step, the flux can liquefy prior to the balls. As the flux liquefies, the balls are free to move and can roll off the bonding site. This can cause missing and mis-aligned balls, and also defective components. Defects can lower throughput in a production process, and necessitate expensive rework procedures.
In order to maintain the balls in alignment with the bonding sites, a ball retaining plate is sometimes employed during the aligning and heating steps. For example, the ball retaining plate can include separate cavities for retaining each solder ball. A vacuum can also be applied to the cavities to provide a positive force for holding the balls in the cavities. U.S. Pat. No. 5,118,027 to Braun et al. discloses a reflow process in which a ball retaining plate and vacuum are used to hold the solder balls.
In general this method, and similar prior art methods, can be performed on balls that have a diameter of about 0.012-in (0.305 mm) or larger. A center to center pitch of the balls can be about 0.018-in (0.457) mm. However, as the balls become smaller, and the spacing between the balls become tighter, it becomes more difficult to align and attach the balls. Accordingly, there is a need in the art for an improved method and apparatus for aligning and attaching balls to substrates of semiconductor components.
Another problem with prior art aligning and attaching methods is the difficulty of fabricating ball retaining plates with the required feature sizes. For example, for fine ball grid array (FGBA) components, the balls can have a diameter as small as 0.005-in (0.127 mm), and a center to center pitch of only about 0.008-in (0.228 mm). It is difficult to make ball retaining plates with the required features sizes using conventional machining processes. Thus there is also a need in the art for improved methods for fabricating ball retaining plates and alignment systems that can accommodate smaller balls and tighter pitches.
In accordance with the present invention, an improved method, apparatus and system for aligning and attaching balls to a substrate are provided. The substrate can comprise a semiconductor component such as a wafer, a die, a chip scale package, or a separate substrate (e.g., BGA substrate) for a semiconductor component. In addition, the substrate can include bonding sites, such as bond pads or land pads, wherein the balls will be attached. Preferably, the balls comprise a eutectic solder material having a relatively low melting point. However, for some applications the balls can comprise a relatively hard metal, such as nickel, copper or beryllium copper.
To perform the method, a ball retaining plate having etched cavities for holding the balls is provided. Preferably the ball retaining plate comprises a material, such as silicon, ceramic, gallium arsenide, or photosensitive glass which can be micromachined with cavities in a dense array using an etching process. With an etching process, each cavity is forms as an etched pocket having sloped sidewalls configured to center and retain a ball. The size and shape of the cavities facilitates loading and retention of the balls on the ball retaining plate.
In addition to the cavities, the ball retaining plate includes vacuum conduits in flow communication with the cavities. The vacuum conduits are adapted for flow communication with a vacuum source for applying a vacuum to hold the balls in the cavities. The ball retaining plate can also include a substrate alignment member, configured to align bonding sites on the substrate to the balls held in the cavities. The substrate alignment member can comprise a separate member attached to the ball retaining plate, or a polymer deposited on the ball retaining plate. Further, the ball retaining plate can be constructed for mating engagement with a ball loader mechanism for loading the balls into the cavities, and a vacuum fixture for applying a vacuum to the cavities.
Initially, the balls are placed in the cavities of the ball retaining plate using the ball loader mechanism. During placement of the balls in the cavities, a vacuum can be applied to the cavities using the vacuum fixture. With the balls held in the cavities by vacuum, the substrate can be aligned with the ball retaining plate using the substrate alignment member. The aligned balls and bonding sites can then be placed in physical contact and the vacuum released. Next, the balls and bonding sites can be heated, such as by placement of the ball retaining plate in a convection furnace. Heating the balls and the bonding sites reflows the balls, and bonds the balls to the bonding sites. With the balls bonded, the ball retaining plate can be withdrawn from the substrate leaving the balls bonded to the substrate.
A system for performing the method includes the ball retaining plate having the micromachined cavities, and the substrate alignment member for aligning the substrate to the cavities. In addition, the system includes the ball loader mechanism for placing the balls in the cavities, and the vacuum fixture for applying a vacuum to the balls held in the cavities. The system also includes a furnace for heating the balls and bonding sites. Preferably the furnace comprises a controlled atmosphere vacuum furnace which is purged of oxygen.
A method for fabricating the ball retaining plate can be performed using a wafer of material. Initially, a laser machining process can be used to form the vacuum conduits in the wafer. A hard mask can then be formed on the wafer with a pattern of openings sized and shaped to form the cavities in flow communication with the vacuum conduits. Using a wet etchant (e.g., KOH or TMAH for a silicon wafer) the wafer can be etched through openings in the hard mask to form the cavities. At the same time the cavities are etched, the laser machined vacuum conduits can also be etched. Next, the hard mask can be stripped, and the wafer sawed to form multiple ball retaining plates each having a required peripheral configuration.